Contents are from Just Ask Susan
"The Dreaded Silica Disease aka "Crusties"
Susan Seitinger

Q: I have been having problems firing my kiln. The porcelain is coming out with white crusty spots on the toes, fingers and sometimes the nose & ears. I've been soft firing my doll parts and using the wet cleaning method... (soaking the sfgw in water prior to cleaning)

A: Your problem sounds like the DREADED SILICA DISEASE caused when wet cleaning the ware when its become too dry during the final stages of cleaning. These white crusty spots can be very frustrating but can be easily eliminated. Most porcelains contain silica which is used to help keep the other clays and ingredients suspended in the liquid state. When greenware is soft fired, the silica forms crystals. As soft fired greenware pieces are soaking in the water, the water begins to draw the silica out to the surface of the ware. During the final stages of cleaning, if the soft fired greenware is allowed to dry too much, you may expose these silica crystals when you are sanding the porcelain away.   Follow these basic steps for dust free cleaning to help with your problem, paying special attention to Step 4.

Step 1: Soak the greenware for a minimum of 10 minutes. I let my soft fired greenware soak at least a half hour in the water and even over night if I haven't finished my cleaning. If your greenware is properly and thoroughly fired, you won't have a problem.

Step 2: In addition to the container used for the initial soaking, fill a dish or pie pan size container with water to do your main cleaning over. Also have a sponge in the water to regularly sponge off the excess paste. Begin cleaning with a Wet scrubber or GreenCleen brush depending on the area to be cleaned. You should not be concerned about doing the fine detail cleaning or any redefining at this time. It is impossible to see any blemishes or scratches through the water and paste. Keep the greenware really wet during the initial cleaning. This will speed up the removal of the main seams.

Step 3: Rinse the ware off thoroughly with your sponge then set aside to dry just long enough for the ware to loose its shine from the water. I usually work on several pieces, so I always have one to work on while I am waiting for another to dry. 
Step 4: When your first piece has lost it's shine it is then possible to see scratches, blemishes and any other marks to remove. Now do the fine finish cleaning. Use a damp nylon stocking along with any other cleaning and detail tools. This is the stage when the silica problems may start if you continue to clean pieces without occasionally resoaking. When soft fired greenware shows signs of becoming too dry and chalky, re-soak briefly then allow the excess water to evaporate before cleaning. The soft fired greenwares high points such as toes, fingers, ears and nose are exactly where you are more than likely to see the crystallization problem. Now that you know the areas to watch for, examine each piece throughly after cleaning and before firing. If the silica is exposed on any of these areas you will see it. It looks like small salt like crystals and will be rougher than other areas. If you see any silica crystals exposed, simply resoak the piecce and reclean that area with a nylon stocking. It is not too difficult to remove this silica, but you must be careful not to remove details or flatten noses. Do not become frustrated with the dust free cleaning. As with any change, it takes time to figure everything out. Some studios will offer a one day cleaning and eye cutting seminar that my help you if you are still having problems. A good hands-on class is definitely worth looking into.
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